This invention relates to an improvement in a magnetic wheel speed sensor.
Magnetic wheel speed sensors are utilized in automotive vehicle wheels to provide electrical signals representing wheel speed. In a typical system, a magnet that is polarized at regular circumferential intervals rotates with the wheel. The magnetic flux issued by the magnet acts upon a sensor, such as a Hall device, that is mounted stationarily adjacent the wheel. As the wheel rotates, the magnetic flux passing through the sensor is caused to vary and as a result the sensor produces a similarly varying electrical output signal. It is only a certain fraction of the flux that actually acts upon the sensor because the remainder of the flux goes astray.
A Hall device is responsive to the intensity of the magnetic flux passing through it, and therefore in a wheel speed sensing system there must be a suitable flux intensity acting upon the device in order for the device to produce a suitable output signal. Accordingly, the magnetomotive force of the magnet, the clearance between the magnet and the Hall device, and the ability to concentrate the flux onto the device are important factors in assuring that a suitable output signal can be developed. The closer the Hall device is to the magnet, the larger the magnetic flux acting on the device; the greater the magnetomotive force of the magnet, the greater the magnetic flux acting on the device. Unfortunately, part manufacturing tolerances limit how small the running clearance between the magnet and the Hall device can be, and more powerful magnets are more costly.
The present invention relates to an improvement for concentrating the magnetic flux on the Hall device while still providing a construction for the sensor that is conducive to economy in manufacture and to satisfactory life and performance in use. Explained in a different way, the invention either makes the running clearance between the magnet and the Hall device less critical for a given magnet strength or else makes the magnet strength less critical for a given running clearance between the magnet and the Hall device.
According to principles of the invention, the sensor is constructed of a magnetically non-conductive housing having a cavity containing a pocket within which the Hall device is disposed. The device is captured within the pocket by means of a potting compound disposed within the housing cavity. Interspersed throughout the potting compound are fragments of magnetically conductive material, such as ferromagnetic filings, chips, powder, flakes, or the like. Such material aids in concentrating the magnetic flux on the Hall device.
The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the following detailed description. Drawings accompany the description and illustrate a preferred embodiment according to the best mode contemplated at the present time for carrying out the invention.